The present invention relates generally to amorphous memory devices and more specifically to an improved structure of amorphous memory devices which minimizes electromigration and retards residual crystal growth.
Amorphous semiconductor memory devices, for example a tellurium base chalcogenide glass, are well known. These memory devices are generally bistable devices having a layer of memory semiconductor material capable of being switched from a stable high resistance condition into a stable low resistance condition when a write or set voltage pulse of relatively long duration (e.g., 1/2-100 milliseconds or more) is applied. Such a set voltage pulse causes current to flow in a small filament (generally under 10 microns in diameter). The set or write current pulse generally heats regions of the semiconductor material above its glass transition and to its crystallization temperature. The consequence is that the material crystallizes around and in the region of the conducting filament. A crystallized low resistance filament remains indefinitely, even when the applied voltage and current are removed, until reset or erased to its initial amorphous high resistance condition.
The set crystallized filament in the semiconductor materials can generally be erased or reset by applying one or more reset current pulses of relatively short duration in the order of 10 microseconds. The reset or erase current pulses heat the entire filament of the semiconductor material to a temperature above the crystallization and melting temperatures of the material. In this condition, the crystalline filament is melted or otherwise reformed into the original amorphous mass. When such a reset current pulse is terminated, the material quickly cools and leaves a generally amorphous composition like the original one. Sometimes, it takes a number of reset current pulses to convert a previous set filament to what appears to be a fully reset state.
While the resistance and threshold voltage values of a reset filament region may indicate it has apparently been fully reset to its original amorphous composition (except for some non-resettable crystallites which ensure that subsequent crystalline filaments are formed in the same place), the reset filament region often is non-homogeneous, with the crystallizable elements like tellurium in various degrees of concentration.
By electromigration during the reset current, the tellurium builds up progressively greater thicknesses of crystalline tellurium next to one of the electrodes involved and therefore progressively reduces the threshold voltage value of the memory switch device. The deposition of tellurium during the reset operations and the preferential ability of tellurium crystallites to grow at the metal/tellurium interface provides nucleation sites which further increase the fractional volume of crystallites in the device. Not only is the threshold value of the device degradated, but sufficient residual crystallization builds up after many set-reset cycles such that the device is continuously on or set and may not be reset.
To prevent threshold degradation due to electromigration, it has been suggested in U.S. Pat. No. 4,115,872 to provide a layer of substantial tellurium between the germanium telluride switchable layer and the positive electrode and a layer of germanium and tellurium with a higher ratio of germanium than the germanium telluride switchable layer between the switchable layer and the negative electrode. Comparison of the threshold voltage stability as a function of the number of set and reset cycles is illustrated in FIG. 5 of the patent.
Although U.S. Pat. No. 4,115,872 directed itself towards preventing electromigration, the enrichment of tellurium at the electrode creates more nucleation sites. This increases the fractional volume of crystallites as more and larger tellurium crystals are formed during operation. This decreases the number of set-reset cycles before lock-on of the device. Thus there exists a need for an amorphous memory device which has substantially stable threshold voltage and an extended life.